Rob Hordijk Twinpeak

Panel design for a Rob Hordijk TWINPEAK filter

The TWINPEAK is a module by Rob Hordijk consisting of two 18dB low-pass filters in parallel, and a built in VCA.

Controls

The module features controls for both filter frequencies, a shared resonance control, modulation depth, and a 'curve' control that blends between low-pass or band-pass outputs. The outputs forth both filters are summed into a single output.

The PEAKING knob functions like 'resonance' on other filters, feeding back to produce a boost in amplitude of of frequencies set by the CUTOFF PEAK 1 and PEAK 2 knobs respectively.

The 1V-OCT CV input will affect both filters equally and allow the filter to track just like an oscillator.

The PEAK 1 MOD and PEAK 2 MOD controls are attenu-verters which have no effect at twelve o'clock, positive effect clock-wise, and a negative or inverted effect on the control signal anti-clockwise.

VCA

The summed output of the filters is normalised to the VCA input. However the VCA can be used independently of the filter and will respond to any uni-polar or 'positive' control voltage.

Phase Inversion

There are separate inputs for each filter, one of which is phase-inverted so their signals are subtracted from each other, instead of added when summed at the FILTER OUT.

In practice this means that if both filters get the same input signal the side with the lowest cutoff frequency is subtracted from the side with the highest cutoff and this creates a band-pass response that only passes what is between the two cutoff frequency settings. The filters can move through each others ranges, meaning it doesn't matter which filter is tuned higher or lower, the output is always the band between the two cutoff settings.

Curve

A flowchart explaining the functionality of the Rob Hordijk TWINPEAK filter

The curve controls set how much of each input is sent to the other filter, which due to their inverted phase and subsequent subtraction from one another produces a band-pass effect.

• Input A is sent to Filter 1 and the Curve A knob sets how much of that signal goes into Filter 2
• Input B is sent to Filter 2 and the Curve B knob sets how much of that signal goes into Filter 1
• The outputs of both filters are then summed at the output

For example if the curve knob is closed the IN A signal only goes into filter one and in the final output you only hear the effect of filter one on the IN A signal. And thus it has a low-pass response as filter two does not get the input signal. That means that there is also not the subtraction at work that create the band-pass, as filter two gets nothing to subtract from the output. Then, by opening the IN A curve knob, the signal level of IN A into filter two is increased and now there is something coming out of filter two and the final output curve changes towards a band-pass curve.

A Rob Hordijk TWINPEAK filter (right) in triple-module format with a DUAL ENV (centre) and OSC HARM (left)

Normalisations

Many of Rob's designs feature internal connections or 'normalisations' between inputs and outputs. These can be broken by inserting jacks into the inputs. The TWINPEAK filter often appeared in a triple-module configuration, featuring some module-to-module normalisations or connections between inputs and outputs.

OSC SYNC / DUAL ENV / TWINPEAK:

• OSC SYNC - SUB SQR OUT > TWINPEAK - IN A
• OSC SYNC - SYNCED OUT > TWINPEAK - IN B
• DUAL ENV - ENV A OUT > TWINPEAK - VCA CV

External Links

• Rob explains the TWINPEAK filter at NOVARS (2017) - https://vimeo.com/240797280
• The Best Filter Ever? by Benoit Faivre